Bump pad structure

ABSTRACT

An embodiment is a bump bond pad structure that comprises a substrate comprising a top layer, a reinforcement pad disposed on the top layer, an intermediate layer above the top layer, an intermediate connection pad disposed on the intermediate layer, an outer layer above the intermediate layer, and an under bump metal (UBM) connected to the intermediate connection pad through an opening in the outer layer. Further embodiments may comprise a via mechanically coupling the intermediate connection pad to the reinforcement pad. The via may comprise a feature selected from the group consisting of a solid via, a substantially ring-shaped via, or a five by five array of vias. Yet, a further embodiment may comprise a secondary reinforcement pad, and a second via mechanically coupling the reinforcement pad to the secondary reinforcement pad.

This application is a continuation of U.S. patent application Ser. No.14/512,895, filed on Oct. 13, 2014, and entitled “Bump Pad Structure,”which is a divisional of U.S. patent application Ser. No. 13/786,045,filed on Mar. 5, 2013, and entitled “Bump Pad Structure,” which is acontinuation of U.S. patent application Ser. No. 12/726,449, filed onMar. 18, 2010, and entitled “Bump Pad Structure,” which claims thebenefit of U.S. Provisional Patent Application Ser. No. 61/176,522,filed on May 8, 2009, and entitled “Bump Pad Structure,” whichapplications are incorporated herein by reference in their entireties.

TECHNICAL FIELD

The present invention relates generally to a bump pad structure for asemiconductor device, and more particularly to a bump pad structure fora flip chip assembly.

BACKGROUND

A flip chip package includes a downward facing semiconductor chipelectrically and mechanically attached to a substrate by solder bumps.Flip chip packages are usually preferred over other conventionalpackages because of the scalability of flip chip packages that allowstheir use in smaller applications. But as the sizes of flip chippackages decrease and as the uses of low-k dielectrics increase,problems with the bump pad metal and low-k dielectric may occur frommechanical stresses on the flip chip package.

FIG. 1A is a cross-sectional view of a solder bump pad and a portion ofits underlying interconnect structure. A solder bump 2 is physicallyconnected to an under bump metal (UBM) pad 4 that is connected to analuminum pad 8 through an opening in an outer passivation layer 6 on asemiconductor chip. The aluminum pad 8 rests on an inner passivationlayer 10. The inner passivation layer 10 is above an undoped siliconglass (USG) layer 12 that is on a low-k dielectric layer 14. The USGlayer 12 may comprise multiple individual metal layers that comprisecircuitry. An aluminum trace 16 electrically couples the aluminum pad 8to an aluminum contact 18 in the interconnect structure. Vias 20 a, 20b, and 20 c through the inner passivation layer 10 connect the aluminumcontact 18 to a contact 22. The contact 22 is coupled to another contact26 through vias 24. Additional contacts and vias may comprise theinterconnect structure as required by the semiconductor device.

FIG. 1B is a layout of the UBM pad 4, the aluminum pad 8, the aluminumtrace 16, the aluminum contact 18, and vias 20 a, 20 b, and 20 c. Theouter octagonal area represents the aluminum pad 8. The middle octagonalarea represents the UBM pad 4. The inner octagonal area represents thedepressed portion of the UBM pad 4.

The bump pad illustrated in FIGS. 1A and 1B are generally used on flipchip assemblies. Flip chip assemblies are preferred in moderntechnologies because of their scalability for use in smallertechnologies. Yet, as the assembly size decreases, particularly to 22 nmtechnology, and as low-k dielectric usage becomes more prevalent,particularly when the k value is less than 2.5, the impact of themechanical stresses arising from the bump pad increases. Stresses on thebump pad, such as peeling or shear forces caused by coefficient ofthermal expansion (CTE) mismatch between the semiconductor device andthe attached package substrate, can cause mechanical failures of thesemiconductor device, such as cracking of the USG, low-k dielectric, orsolder bump, due to the weakened bump pad.

Another structure commonly used in flip chip technology is the directbump on copper (DBOC) structure. In the DBOC structure, the UBM is indirect contact with the copper metal of the top metallization layer. Noaluminum pad or inner passivation layer is used in the DBOC structure.Without an aluminum pad or an inner passivation layer to act as abuffer, a DBOC structure generally has less mechanical strength andsuffers from the same problems as discussed above. Accordingly, there isa need in the prior art for a bump pad with increased mechanicalstrength to overcome the deficiencies of the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawing, in which:

FIG. 1A is a cross-sectional view of a conventional bump pad structureand interconnect;

FIG. 1B is a plan view of a conventional bump pad structure andinterconnect;

FIG. 2 is a cross-sectional view of a bump pad structure in accordancewith an embodiment of the present invention;

FIG. 3 is a cross-sectional view of a bump pad structure in accordancewith a further embodiment of the present invention;

FIG. 4 is a chart comparing the stresses at the interface of the undopedsilicon glass (USG) layer and the low-k dielectric layer between theconventional structure and embodiments of the present invention;

FIG. 5 is a chart illustrating the effects of the dimensions of thecopper pad on the stresses at the interface of the USG layer and thelow-k dielectric layer;

FIGS. 6A through 6D are plan views of vias and a bump pad structure inaccordance with embodiments of the present invention;

FIG. 7 is a chart illustrating how different via layouts affect thestress at the interface of the USG layer and the low-k dielectric layer;

FIG. 8 is a cross-sectional view of a bump pad structure in accordancewith a further embodiment of the present invention; and

FIGS. 9A through 9M illustrate a process to create a bump pad structurein accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the present embodiments are discussed in detailbelow. It should be appreciated, however, that the present inventionprovides many applicable inventive concepts that can be embodied in awide variety of specific contexts. The specific embodiments discussedare merely illustrative of specific ways to make and use the invention,and do not limit the scope of the invention.

The present invention will be described with respect to embodiments in aspecific context, namely a bump pad for a flip chip assembly. Theinvention may also be applied, however, to any packaging assembly thatuses solder bump pads, such as a DBOC structure.

FIG. 2 illustrates a first embodiment of the invention. A solder bump 30is physically connected to an under bump metal (UBM) pad 32 that isconnected to an aluminum pad 36 through an opening in an outerpassivation layer 34 on a semiconductor chip. The aluminum pad 36 restson an inner passivation layer 38. The inner passivation layer 38 isabove an undoped silicon glass (USG) layer 42 that is on a low-kdielectric layer 44. A solid copper pad 40 is on the USG layer 42beneath the aluminum pad 36. The low-k dielectric layer 44 may alsocomprise circuitry 46. The layers may also comprise any other knownconfiguration or material. For example, the low-k dielectric layer 44may instead be another USG layer. Further, not shown in FIG. 2, andsubsequent FIGS. 3 and 8, is an aluminum trace electrically coupling aninterconnect structure to the aluminum pad 36.

FIG. 3 represents another embodiment. FIG. 3 is the same structure asFIG. 2, except the structure in FIG. 3 has vias 48 connecting thealuminum pad 36 to the copper pad 40.

These embodiments reduce the stresses on the USG layer 42 and low-kdielectric layer 44. FIG. 4 is a chart illustrating the reduction ofstresses at the interface of the USG layer 42 and the low-k dielectriclayer 44. The conventional aluminum pad is represented in FIG. 1A. Thealuminum pad with a copper pad is illustrated in FIG. 2, and thealuminum pad with the copper pad and via connection is in FIG. 3. Thestresses on the interface are normalized using the conventional pad as abaseline. The aluminum pad and copper pad structure (FIG. 2) had areduction in peeling stress of nine percent and in shear stress oftwelve percent from the normalized baseline. The aluminum pad and copperpad with vias structure (FIG. 3) had a reduction in peeling stress offifteen percent and in shear stress of twenty-two percent from thebaseline.

The reduction in stresses on the interface is generally caused by anincrease in the Young's modulus caused by adding the copper pad 40. TheYoung's modulus of the low-k dielectric is approximately 10 GPa, and theYoung's modulus of the USG is approximately 70 GPa. However, the Young'smodulus of copper is approximately 128 GPa. Accordingly, insertingcopper into the structure will generally increase the Young's modulus ofthe structure to increase the mechanical strength to better protect theinterface.

Not only can the type of structure used affect the stresses on the USGlayer 42 and the low-k dielectric layer 44, but also the dimensions ofthe copper pad 40 can affect the stresses. FIG. 5 is a chart of how thedimensions of the copper pad 40 can affect the stresses. The x-axis ofthe chart is delta Δ in micrometers. Delta Δ represents the differenceof the circumradius of the UBM pad 32 from the circumradius of thecopper pad 40 as illustrated in FIG. 3. In FIG. 5, the stresses on theinterface of the USG layer 42 and the low-k dielectric layer 44 arenormalized to a baseline of when the cirumradius of the copper pad 40equals the circumradius of the UMB pad 32 such that delta Δ is zero. Asthe chart shows, the stresses decrease as delta Δ increases, yet thedecrease in normalized stress is relatively small once delta Δ exceedsfive micrometers. Thus, the delta Δ can be said to become saturated atfive micrometers. Accordingly, delta Δ is preferred to be about fivemicrometers although delta could be any possible size.

Further, the layout of the vias 48 used to connect the aluminum pad 36to the copper pad 40 can affect the stresses on the USG layer 42 and thelow-k dielectric layer 44. FIGS. 6A through 6D are exemplary layouts ofvias 48. FIG. 6A shows a layout with a solid octagonal via 48 a. Thedashed line represents the layout of the UBM pad 32, and the solid lineis the copper pad 40. Generally, the circumradius of the UBM pad 32 isbetween approximately seventy-five and one hundred twenty micrometers,and the circumradius of the aluminum pad 36 (not shown) is approximatelyfour micrometers larger than the circumradius of the UBM pad 32. FIG. 6Bshows an octagonal ring via 48 b in which the outer circumradius of thevia 48 b is ten micrometers larger than the inner circumradius of thevia 48 b. FIG. 6C shows an octagonal ring via 48 c in which the outercircumradius of the via 48 c is twenty micrometers larger than the innercircumradius of the via 48 c. The vias in FIGS. 6B and 6C have surfaceareas of 28.4 percent and 52.1 percent, respectively, of the surfacearea of the aluminum pad 36. Further, the difference of the inner andouter circumradii of the octagonal ring vias 48 b and 48 c may beincreased or decreased, such as to five micrometers or to twenty-fivemicrometers. With a five micrometer difference, the surface area of thevia is 14.8 percent of the aluminum pad 36, and with a twenty-fivemicrometer difference, the surface area of the via is 62.1 percent ofthe aluminum pad 36. FIG. 6D shows a five by five array of vias 48 b.

FIG. 7 is a chart illustrating how these different layouts affect thestress at the interface of the USG layer 42 and the low-k dielectriclayer 44. The solid octagonal via 48 a in FIG. 6A is used as a baseline,and all other layouts are normalized to it. From the chart, one can seethat the normalized stresses decrease from the baseline to the twentymicrometer octagonal ring via 48 c in FIG. 6C to the five by five arrayof vias 48 d in FIG. 6D to the ten micrometer octagonal ring via 48 b inFIG. 6B.

The disadvantage of the foregoing embodiments is that more area isneeded for the copper pad 40 in the USG layer 42 or that an extra metallayer must be included in the USG layer 42. As compared to the bump padin FIG. 1A, embodiments of the invention would require an area on thetop metal layer within the USG layer 42 in which to place the copper pad40. This may necessitate re-routing circuitry on the top metal layer toclear the area for the copper pad 40. Alternatively, an extra metallayer could be added within the USG layer 42 in which to place thecopper pad 40. This would not require re-routing circuitry in anexisting semiconductor device design, but would cost more in terms ofadded processing and materials to add the extra metal layer.

FIG. 8 is another embodiment similar to that illustrated in FIG. 3,except a second copper pad 52 is under the USG layer 42 and on the low-kdielectric layer 44. Vias 50 connect the copper pad 40 to the secondcopper pad 52. The vias 50 may be one or many individual vias or may bea via in accordance with the layouts of vias in FIGS. 6A through 6D.Further, more copper pads may be added to the structure in FIG. 8. Thecopper pads may be within the multiple metal layers that comprise theUSG layer 42 or may be within the USG layer 42 and the low-k dielectriclayer 44. The multiple copper pads may be connected by vias or may notbe connected. The addition of the second copper pad 52 further increasesthe Young's modulus of the structure, thus increasing the overallmechanical strength of the structure.

Other features of embodiments include a thicker aluminum pad 36 and/oraluminum trace, a thicker USG layer 42, or merely having vias 48 throughthe inner passivation layer 38 without an underlying copper pad.Generally, an aluminum pad 36 is about 1.45 μm thick. Increasing thisthickness, for example to 2.5 μm, may increase the mechanical strengthof the structure and will protect the USG layer 42 and low-k dielectriclayer 44 more. Likewise, increasing the thickness of the USG layer 42may increase the mechanical strength of the USG layer 42 to increasinglyprotect the USG layer 42 and the low-k dielectric layer 44. The USGlayer 42 may be thickened by increasing the thickness of existing layerswithin the USG layer 42 or by adding new layers within the USG layer 42.Further, vias 48 through the inner passivation layer 38 alone without anunderlying copper pad may increase the mechanical strength of thestructure over conventional bump pads.

FIGS. 9A through 9M illustrate a process to build the bump pad structurein FIG. 3 according to another embodiment. In FIG. 9A, the USG layer 100is formed on a low-k dielectric layer (not shown). In FIG. 9B, aphotoresist layer 102 is patterned on the USG layer 100 to expose theUSG layer 100 where interconnect via openings 104 are to be etched.Then, the interconnect via openings 104 are etched using knownphotolithography techniques. In FIG. 9C, the interconnect via openings104 are then partially filled with plugs 106 and the photoresist layer102 is removed. In FIG. 9D, another photoresist layer 108 is formed onthe USG layer 100 and patterned to expose the USG layer 100 where aninterconnect contact is to be formed over the interconnect via openings104 and where a copper pad is to be formed. The USG layer 100 is thenetched to a depth of at least to the top of the plugs 106 using knownphotolithography techniques.

In FIG. 9E, the photoresist layer 108 and plugs 106 are removed. Next,copper 110 is deposited on the USG layer 100. A diffusion barrier layermay be formed using physical vapor deposition before depositing thecopper 110. Also, a copper seed layer may be formed for electroplatingafter forming the diffusion barrier layer but before depositing thecopper 110. In FIG. 9F, any excess copper 110 is removed, such as bychemical mechanical polish. This forms the interconnect vias 112, theinterconnect contact 114, and the copper pad 116. Alternatively,conventional dual damascene process steps may be employed to form theinterconnect vias 112, the interconnect contact 114, and the copper pad116.

In FIG. 9G, a first passivation layer 118 is deposited over the USGlayer 100. In FIG. 9H, a photoresist layer 120 is formed on the firstpassivation layer 118. The photoresist layer 120 is patterned to exposeportions of the first passivation layer 118 that overlie theinterconnect contact 114 and the copper pad 116. The first passivationlayer 118 is then etched using known photolithography techniques leavingvia openings 122 and 124. In FIG. 9I, the photoresist layer 120 isremoved and aluminum 126 is deposited. The aluminum 126 fills the viaopenings 122 to form aluminum interconnect vias 128 and fills the viaopenings 124 to form aluminum vias 130. In FIG. 9J, a photoresist layer132 is formed over the aluminum 126 and patterned to form an aluminuminterconnect contact 134, an aluminum trace 136, and an aluminum pad138. The aluminum 126 is then etched to form those components.

In FIG. 9K, the photoresist layer 132 is removed, and a secondpassivation layer 140 is deposited on the structure. In FIG. 9L, aphotoresist layer 142 is formed on the second passivation layer 140 andpattered to expose a portion of the second passivation layer 140 abovethe aluminum pad 138. The second passivation layer 140 is then etcheddown to the aluminum pad 138 leaving a UBM opening 144. In FIG. 9M, thephotoresist layer 142 is removed, and a UBM pad 146 is formed in the UBMopening 144 and is connected to the aluminum pad 138.

In accordance with an embodiment of the present invention, a bump bondpad structure comprises a substrate comprising a top layer, areinforcement pad disposed on the top layer, an intermediate layer abovethe top layer, an intermediate connection pad disposed on theintermediate layer, an outer layer above the intermediate layer, and anunder bump metal (UBM) connected to the intermediate connection padthrough an opening in the outer layer.

In accordance with another embodiment of the present invention, a bumpbond pad structure comprises a copper pad on a top layer of a substrate,an aluminum pad on an inner passivation layer, a via mechanicallycoupling the copper pad to the aluminum pad, and an UBM mechanically andelectrically coupled to the aluminum pad through an opening in an outerpassivation layer. The inner passivation layer is on the top layer ofthe substrate.

In accordance with another embodiment of the present invention, a methodfor creating a bump bond pad structure, the method comprises forming areinforcement pad on a top layer of a substrate, forming an intermediatelayer on the top layer of the substrate, forming an intermediateconnection pad on the intermediate layer and a via through theintermediate layer to couple the intermediate connection pad to thereinforcement pad, forming an outer layer over the substrate, andforming an UBM in an opening of the outer layer to couple the UBM to theintermediate connection pad.

An advantage of an embodiment of the present invention is generally areduction in stresses on an interface between an USG layer and a low-kdielectric layer caused by an increase in the Young's modulus of thestructure. The increase in the Young's modulus generally causes theentire structure to have more mechanical strength.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims. For example,although many of the feature of the embodiments describe above havecomprised either copper or aluminum, each feature may comprise copperinstead of the described aluminum, or vice versa. As another example, itwill be readily understood by those skilled in the art that the layersdescribed above, i.e. the passivation layers, the USG layer, and thelow-k dielectric layer, are not necessarily the layers present in astructure in accordance with the foregoing embodiments.

Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, composition of matter, means, methods and steps describedin the specification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present invention, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present invention. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, or steps.

What is claimed is:
 1. A structure comprising: a substrate comprising afirst dielectric layer; a reinforcement pad disposed in the firstdielectric layer, the reinforcement pad having first outer edgesconfigured in a first shape in a first plane parallel to a top surfaceof the first dielectric layer, the first shape having a firstcircumradius; a second dielectric layer over the first dielectric layer;an intermediate connection pad disposed on the second dielectric layer;a passivation layer over the second dielectric layer; and an under bumpmetal (UBM) connected to the intermediate connection pad through anopening in the passivation layer, the UBM having second outer edgesconfigured in a second shape in a second plane parallel to the topsurface of the first dielectric layer, the second shape having a secondcircumradius, the first circumradius exceeding the second circumradiusby at least 4.167%.
 2. The structure of claim 1 further comprising: atleast one first via in the second dielectric layer, the at least onefirst via directly connecting the reinforcement pad and the intermediateconnection pad.
 3. The structure of claim 2, wherein the at least onefirst via consists of a single solid via in the second dielectric layerbetween the intermediate connection pad and the reinforcement pad. 4.The structure of claim 2, wherein the at least one first via comprises asubstantially ring-shaped via.
 5. The structure of claim 2, wherein theat least one first via comprises an array of discrete vias.
 6. Thestructure of claim 1 further comprising: a secondary reinforcement pad,wherein the substrate further comprises a third dielectric layer underthe first dielectric layer, the secondary reinforcement pad beingdisposed in the third dielectric layer.
 7. The structure of claim 6further comprising: a first via connected between the intermediateconnection pad and the reinforcement pad, the first via being in thesecond dielectric layer; and a second via connected between thereinforcement pad and the secondary reinforcement pad, the second viabeing in the first dielectric layer.
 8. The structure of claim 1,wherein the reinforcement pad comprises copper, and wherein theintermediate connection pad comprises aluminum.
 9. The structure ofclaim 1, wherein the first circumradius is at least 5 μm larger than thesecond circumradius.
 10. A structure comprising: a first metal pad overa substrate; a first passivation layer over the first metal pad, atleast one via in the first passivation layer; a second metal pad on thefirst passivation layer, the second metal pad having a differentmaterial composition than the first metal pad, the at least one viadirectly connecting the first metal pad and the second metal pad; asecond passivation layer over the first passivation layer; and an underbump metal (UBM) connected to the second metal pad through an opening inthe second passivation layer.
 11. The structure of claim 10, wherein acircumradius of the first metal pad is at least 5 μm larger than acircumradius of the UBM.
 12. The structure of claim 10, wherein thefirst metal pad comprises copper, and wherein the second metal padcomprises aluminum.
 13. The structure of claim 10, wherein the at leastone via: is a substantially ring-shaped via, is an array of individualvias, or consists of a single solid via between the first metal pad andthe second metal pad.
 14. A method comprising: forming a first materialin a first recess of a first dielectric layer to form a reinforcementpad, the reinforcement pad having a first circumradius; forming a seconddielectric layer over the first dielectric layer; forming a secondmaterial on the second dielectric layer to form an intermediateconnection pad; forming a passivation layer over the intermediateconnection pad; forming an opening in the passivation layer to theintermediate connection pad; and forming an under bump metal (UBM) inthe opening and partially on the passivation layer, the UBM having asecond circumradius, the first circumradius exceeding the secondcircumradius by at least 4.167%.
 15. The method of claim 14, wherein thereinforcement pad comprises first outer edges, the first outer edgesforming a first shape in a first plane parallel to a top surface of thefirst dielectric layer, and the UBM has second outer edges, the secondouter edges forming a second shape in a second plane parallel to the topsurface of the first dielectric layer, the second shape corresponding tothe first shape.
 16. The method of claim 14, wherein the first materialcomprises copper, and the second material comprises aluminum.
 17. Themethod of claim 14 further comprising: etching a first opening throughthe second dielectric layer to the reinforcement pad, the secondmaterial being formed in the first opening to form a first via.
 18. Themethod of claim 17, wherein the first opening is an array of individualopenings or consists of a single opening between the reinforcement padand the intermediate connection pad.
 19. The method of claim 14 furthercomprising: etching a third dielectric layer to form a second recess;forming a third material in the second recess to form a secondaryreinforcement pad; forming the first dielectric layer over the thirddielectric layer; and etching a second opening through the firstdielectric layer in the first recess to the secondary reinforcement pad,wherein forming the first material in the first recess furthercomprising forming the first material in the second opening to form asecond via.
 20. The method of claim 14, wherein the first circumradiusis at least 5 μm larger than the second circumradius.